1. Field of the Invention
The present invention relates to a method for forming a metal film on the surface of a substrate metal, which comprises soaking a substrate metal, under a heating condition, in a solution of a salt of the metal to be coated on the substrate metal. More particularly, the present invention relates to a new method for forming a film of a soldering metal on the surface of a substrate metal, such as a printed circuit on the surface of an insulation panel, which comprises soaking the substrate metal in a solution of a specific salt of the soldering metal under a heating condition.
2. Description of the Prior Art
In general, electronic circuits are manufactured by printing a pattern of a substrate metal on a panel, forming a film of a soldering metal such as a soldering alloy on the substrate metal, laying circuit parts on the metal film and heating the whole to solder the parts and the metal film.
Heretofore, a pasty solder is known for such a purpose, which is a pasty dispersion of a powder of a soldering alloy and a flux in a high boiling point solvent such as a mineral spirit functioning as a viscosity-regulating vehicle and is used for forming a film of the soldering alloy on the substrate metal. The use of such a pasty solder for forming a film of a soldering alloy on a substrate metal of a circuit panel generally necessitates the steps of applying the pasty solder in a given configuration onto the surface of a panel having electronic parts thereon by way of screen printing or with the aid of a dispenser and heating panel to melt the powder of the soldering alloy and attach it onto the surface of the substrate metal by a cooperative action of the flux. When the pasty solder is heated, the powder of the soldering alloy is molten and any oxide existing on the surface thereof is reduced by the action of the flux to form pure molten soldering alloy particles having a clean surface. At the same time, the substrate metal on the panel is also reduced to have a clean surface. Thus, the molten soldering alloy particles are improved in their coagulating force and wettability for the substrate metal, and as the result, a film of the soldering alloy is formed on the surface of the substrate metal.
In such a pasty solder, the soldering alloy exists in the form of discrete free alloy particles separated from the flux and the solvent. The size of the particles may be very small but is still too large to form a homogeneous composition together with the flux and the solvent. Many attempts are still being made to minimize the size of the alloy particles but the size is substantially limited at present to about 10.mu. in diameter. The alloy particles should have a diameter of less than 1.mu. to form a stable homogeneous pasty solder.
In recent years, the density of printed circuits for electronic equipments becomes higher which necessitates the circuit patterns on substrates to be constructed by a number of extremely thin lines. In some cases, printed circuits are constructed by a circuit pattern having lines of several hundred microns in width at similar intervals. As the density of circuit patterns becomes higher, a pasty solder therefor should correspondingly be supplied in a more precise manner. In the case in which a pasty solder is supplied by way of screen printing for the manufacture of such high density printed circuits, a screen having a smaller mesh should be used. In the case wherein a dispenser is used for supplying a pasty solder, the use of an extrusion nozzle having a smaller diameter becomes necessary as well. Since solid particles of the soldering alloy are contained in the pasty solder as described above, the size of the particles cannot be disregarded at the time of supplying the pasty solder to substrates by way of screen printing or with the aid of a dispenser. In the case of supplying a pasty solder by way of screen printing, the solid particles of the soldering alloy contained therein often cause clogging of the screen so that it becomes difficult to supply the alloy particles homogeneously in a given configuration. Further, the friction between the alloy particles and the screen may cause damage of a mask, thus shortening the life of the screen. In the case of using a dispenser for supplying a pasty solder, the alloy particles would clog the extrusion nozzle of the dispenser whereby the clogging particles would function as a filter for the pasty solder passing through the nozzle which would permit passage of only the flux and the solvent while leaving the solid particles of the soldering alloy. This would make it extremely difficult to extrude the pasty solder wherein the alloy particles are homogeneously dispersed.
Soldering between the alloy particles and the substrate metal is attained, as described above, by a mutual coagulation force and an enhanced substrate metal-wetting property of the molten alloy particles. Since the surface tension of the individual molten alloy particles is strong, it is difficult to allow all of the alloy particles to participate in soldering. The alloy particles not participating in the soldering, would remain on the substrate panel and cause an occurrence of unexpected short circuits between the lines of the circuit pattern. Thus, a short circuit may easily be formed by the existence of even a single alloy particle between the lines constituting the circuits, which would result in a detrimental defect. To avoid such a result, it is recommended that the printed circuits be washed after soldering with an organic solvent to eliminate any residual alloy particle. Even by such washing, it is difficult to eliminate the residual alloy particles entirely from the treated circuits.
Such a drawback may be overcome more or less by reducing the diameter of the particles of the soldering alloy used. However, the reduction in the diameter of the particles makes the production cost higher and the particles in the resultant pasty solder so stable that coagulation of the alloy particles may hardly occur and would make the soldering operation itself difficult.
In order to prevent any cohesion of the alloy particles to undesirable areas which causes the formation of a short circuit. It has also been put into practice to mask with a resist all areas other than the area on which the formation of a metal film is desired, thereby forming the solder film exclusively on the desired area. Noteworthy herein is that the size of the area on which the deposition of the alloy particles is desired is almost equal to the diameter of the individual alloy particles. Thus, it is extremely also difficult in this case to supply the alloy particles precisely to the desired area for the formation of a sufficiently thick film of the soldering alloy onto the desired area.
As further means for forming a metal film on the desired areas of the substrate, a panel carrying the substrate metal thereon is masked in the undesired areas with a resist and is dipped in a molten solder bath. However, by this means, it is also extremely difficult to form a film of the soldering alloy precisely in compliance with the complicate pattern.
The above mentioned various drawbacks result apparently from their solid-liquid two phase compositions wherein solid particles of the soldering alloys are dispersed in a liquid or sol medium. It is surmised therefore that these drawbacks will be overcome if the solders are of a single homogeneous phase. Thus, in the field of solders which are used in the electronic industry, there is room for improvement in the conventional pasty solders, especially in compositions thereof. Thus, there is a strong demand for developing a new type of solder for electronic printed circuits which would overcome these drawbacks.